A sample glass of Lake Erie water is photographed near the city of Toledo water intake crib on Aug. 3 in Lake Erie. / Haraz N. Ghanbari, AP

by Jolie Lee, USA TODAY Network

by Jolie Lee, USA TODAY Network

Nearly half a million people in northwestern Ohio were without drinking water over the weekend after toxins produced by algae were found in the water supply.

The mayor of Toledo lifted the ban on drinking city tap water Monday.

Algae is nothing new in Lake Erie, but why are the toxins so bad now and could this happen again?

What is contaminating the water? Isn't algae harmless?

Ohio officials detected above-normal levels of toxins called microcystins produced by a type of blue-green algae. Algae occurs naturally in the lake and most is harmless. But when toxins are released, they can exist for weeks or even months, according to the U.S. Environmental Protection Agency. Microcystins are not regulated by the EPA.

What does fertilizer have to do with harmful blooms?

In general, the growth of algae that releases toxins are referred to as harmful algal blooms. In Ohio's case, the algae thrives on an overabundance of phosphorous. The main sources of this chemical include fertilizers, failing septic tanks and power plant emissions, according to the Ohio Environmental Protection Agency.

In particular, fertilizer has contributed to increased levels of dissolved phosphorous in Ohio's rivers, said Laura Johnson, a research scientist at the National Center for Water Quality Research at Heidelberg University in Tiffin, Ohio.

Changing agricultural practices have led to the higher phosphorous levels. Since the mid-1990s, farms got bigger and started to "broadcast" fertilizing, which does not get fertilizer very deep into the soil, Johnson said.

Farmers also turned to no-till farming to control erosion, but the trade-off was increased run-off from the fertilizer, she said.

Over the same period, rain storms have increased, she said.

Sure, the ban is lifted, but could toxins contaminate the water again?

"It's very possible," Johnson said. Usually the blooms are at their most intense in September. "What's really scary is it's so early in the season," she said.

Officials will likely take extra precautions to filter the water from now on, Johnson said.

What are possible solutions?

Soil in the region contains a lot of clay. The National Center for Water Quality Research is promoting increased use of organic matter in the soil, which acts "more like a sponge and will hold onto water better," Johnson said.

What if you swallow the water?

A person can get the toxin into their system when swallowing or touching the water, according to the Ohio EPA. With most microcystins, the primarily target is the liver, according to the U.S. EPA. Symptoms include nausea, vomiting, diarrhea and upset stomach.

In western Lake Erie, NOAA predicted "significant" blooms this summer. The harmful algal blooms were common in Lake Erie between the 1960s and 1980s, but after a 20-year hiatus, the blooms have been increasing over the past decade, according to NOAA.